FRAM (ferroelectric random access memory) devices have been widely used because of such desirable performance characteristics as non-volatility, high speed, large capacity and low power-consumption.
Bismuth titanate(BTO, Bi4Ti3O12) which undergoes phase-transition at its Curie temperature of 675° C. is a well-known ferroelectric material for the FRAM device. However, this material exhibits high electric fatigue, that is, the residual polarization value decreases after a number of depolarization cycles. Further, a strontium bismuth tanthalate (SBT, SrBi2Ta2O9) layer which is also known as a ferroelectric material requires a high crystallization temperature of 800° C. or higher although it has good electric fatigue property, and a lead zirconium titanate (PZT, PbZrxTi1-xO3(0<x<1)) layer having a relatively low crystallization temperature exhibits early fatigue only at 106 cycles.
Furthermore, it was recently reported by the present inventors that a bismuth lanthanium titanate (BLT, Bi4-xLaxTi3O12(0<x<4)) layer needs a relatively low crystallization temperature of around 700° C., and that it exhibits no sign of fatigue even after 1010 cycles (see [B. H. Park, et al., Nature, 401(14), 682(1999)]).
The present inventors have further studied to develop a novel ferroelectric thin film, and as a result, found that the doping with yttrium (Y) can improve the ferroelectric properties of the BTO composition because an oxide film of Y has a lower heat (-1815 kJ/mole) of formation than that (-1703.2 kJ/mol) of lanthanium (La).